The proposed studies will examine the role of medullary toxicity in the determination of urinary sodium excretion. Medullary toxicity will be manipulated by 1) exposure of the extrarenal papilla, 2) water diuresis, and 3) superfusion of the exposed papilla with a hypertoxic urea-NaCl solution. In each of these conditions delivery of sodium and chloride to the bend of Henle's loop of juxtamedullary nephrons and the late distal tubule of superficial nephrons and the base and tip of the papillary collecting duct will be evaluated during hydropenia and after either 2% body weight Ringer solution or benzolamide administration.